Fused silica and ultra low expansion glass production furnaces produce boules by the deposition of soot or silica particles produced from vaporous or liquid reactants and consolidation of the soot. Such metal oxide soot can be produced by flame hydrolysis of precursors in the form of a vapor or atomized liquid carried by a carrier gas into the flame of a burner or multiple burners. The furnace is surrounded by refractory materials. Refractory materials in these furnaces are exposed to temperatures exceeding 1650° C.
A commercial application of flame hydrolysis involves forming and depositing fused silica soot to form large fused silica containing bodies or boules that may also include dopants added to the fused silica body. These boules may be used individually, or they may be finished into optical members such as lenses, prisms, mirrors, etc. which may be integrated into optical equipment.
FIG. 1 shows a furnace 100 for producing fused silica or ultra low expansion glass. The furnace includes a crown 12 and a plurality of burners 14 projecting from the crown. As noted above, silica particles are generated in a flame when a silicon containing raw material together with a natural gas are passed through the plurality of burners 14 into the furnace chamber 26. These particles are deposited on a hot collection surface of a rotating body where they consolidate to the solid, glass state. The rotating body is in the form of a refractory cup or containment vessel 15 that includes a collection surface 21 and may include lateral walls 17 which surround the boule 19 and provide insulation to the glass as it builds up. The refractory insulation ensures that the collection surface and the crown are kept at high temperatures.
A standard fused silica or ultra low expansion glass production furnace further includes a ring wall 50 which supports the crown 12. The furnace further includes a rotatable base 18 mounted on an oscillation table 20. The base is rotatable about an axis 3. The crown 12, the ring wall 50, the base 18 and the lateral walls are all made from suitable refractory materials, typically zircon refractory materials. The crown in particular is made from sintered, porous zircon refractory bodies in the form of bricks. The bricks making up the crown are arranged in the form of an arched surface, and the individual bricks are usually held into place with a mechanical fastener. One disadvantage associated with using bricks is that each individual brick and fastener is a possible failure point in the furnace crown.
The crown of the furnace is typically made from zircon refractory bricks made from a batch containing primarily milled zircon powder and a burnout material to provide a porous brick. To produce a zircon batch, minor amounts of dispersant and binder are thoroughly mixed in water. The burnout material and the zircon powder are added to the solution in a mixer to produce a pourable slurry. The slurry is poured into molds, and the green bodies produced by this method are dried and fired to release the burn out material and sinter the zircon body. One disadvantage of this process is that the pore size, pore structure and pore distribution are difficult to control. Although the process provides a porous refractory brick, it is difficult to form a network of interconnected pores in the brick. Another disadvantage of this process is that it is difficult to obtain a pore surface area greater than 0.5 m2/g, as measured by the BET method. It is also difficult to produce a brick having porosity greater than 50%, and the density of bricks produced by this method exceeds 2 g/cm3. A brick having a higher porosity and a lower density would allow for the production of more lightweight crown materials. Moreover, it is possible to produce larger refractory shapes and fewer failure points across a furnace structure such as a crown.
Zircon refractories used in fused silica furnace must contain low levels of metallic impurities, and one way of reducing the levels of impurities is through a halogen gas treatment process, which is described in U.S. Pat. No. 6,174,509. Although the process for treating zircon refractories described in U.S. Pat. No. 6,174,509 produces refractories that have a substantially lower levels of metallic impurities than untreated zircon refractories, there continues to be a need for refractory materials that contain and thus introduce even lower levels of impurities to materials produced in the furnace.
It would be desirable to provide a refractory having a high pore surface area and low density, enabling the formation of lightweight refractory shapes larger than conventional refractory bricks. It would also be advantageous to provide a refractory material having controlled pore size and distribution, particularly a refractory material having a network of interconnected pores.